feat(api,ui): cart + bulk-print across integrations

[strausmann]

Implements ADR 0013 — cart + bulk-print part.

Backend

• POST /api/print/{printer_id}/batch with body:

  {
    "items": [
      {"integration": "snipeit", "lookup_id": "ASSET-12345", "layout_id": "<uuid>", "quantity": 3},
      {"integration": "grocy",   "lookup_id": "42",          "layout_id": "<uuid>", "quantity": 1}
    ]
  }

• Returns single job_id; the job is a multi-page raster stream (Brother native: 0Ch between pages, 1Ah after last)
• Each item is rendered once and the resulting page is repeated quantity times in the stream
• Validates that every item's layout matches the printer's tape width — rejects 422 if mixed
• Layout compatibility: must match printer-model filter if set on the layout
• quantity defaults to 1 if omitted; valid range 1-99

Frontend

• Cart state persisted in localStorage (key lph.cart), schema:

  {
    "version": 1,
    "items": [
      {"id": "ulid", "integration": "snipeit", "lookup_id": "...", "layout_id": "...",
       "quantity": 3, "title_snapshot": "...", "thumbnail_url": "...", "added_at": "..."}
    ],
    "updated_at": "..."
  }

• Cart icon top-right (printer + badge), badge counter = sum of all quantity values
• Cart icon visible from every view (sticky in app shell)
• "Add to cart" button on the item detail view
• Cart screen: items grouped by integration; per item: thumbnail, title, ID, layout dropdown, tape (auto from layout), quantity stepper (− / +) plus direct number input, remove button
• Cart total summary: total label count, estimated tape consumption per tape size, target-printer breakdown
• "Print all" → confirmation modal showing the batch breakdown (printer × tape × N items) → submits one batch per (printer, tape_mm) group
• "Clear cart" secondary button (with confirmation)
• Empty state with link back to browse mode

Acceptance

• Cart survives page reload (localStorage)
• Quantity stepper enforces 1-99 range
• Cart icon badge updates immediately on add/remove/qty-change
• Multi-tape cart shows correct grouping in the confirmation modal
• Single-group cart submits exactly one job
• Layout-mismatch rejected at API level (422 with clear message)
• Print job total page count = sum of all quantities (verified in tests)

Out of scope (separate issue if requested)

• Server-side cart sessions for cross-device persistence (Option D in ADR 0013)
• "Save cart for later" / named carts